Reaction kinetics and mechanisms of inorganic hydrides on germanium surfaces

Author

Cohen, Stephen Michael

Date

1992

Degree

Doctor of Philosophy

Abstract

The surface reactivity of germanium is of interest because of novel Si$\sb{x}$Ge$\sb{1-x}$ heterostructure applications and the insight into semiconductor surface chemistry attainable through comparative studies on silicon and germanium. The adsorption of the inorganic hydrides H$\sb2$S, H$\sb2$O, NH$\sb3$, and HX (X = Cl, Br) on Ge(100) was investigated by temperature-programmed desorption (TPD) for the first time. The initial sticking probability $S\sb0$ for H$\sb2$S exhibits at most only a minor temperature effect, remaining roughly constant at $\sim$0.23 within the temperature range 173 K $\le$ T $\le$ 373 K. Adsorbed H$\sb2$S decomposes upon heating into H$\sb2$ and GeS, which desorb at 570 K and 660 K, respectively. The initial sticking probability for H$\sb2$O depends strongly upon substrate temperature, dropping from 0.28 at 173 K to $\sim$0.02 at 273 K. Adsorbed water decomposes to yield H$\sb2$ and GeO, which desorb at peak temperatures of 570 K and 660 K, respectively. Both HCl and HBr adsorb on Ge(100), and desorb upon heating to 570 K as H$\sb2$ and to 580 K as HX in roughly equal proportions. The remaining X(a) desorbs as GeX$\sb2$ at 680 $\sim$ 760 K. The initial sticking probability for HBr is 0.77 at 273 K, falling slightly to 0.55 at 373 K, whereas S$\sb0$ for HCl falls from 0.30 at 273 K to 0.066 at 373 K. The peak shapes and independence of peak temperature with coverage for H$\sb2$ and HX for all adsorbates indicate approximately first-order desorption kinetics, but GeX$\sb2$ desorption follows second-order desorption kinetics. Adsorption of NH$\sb3$ was not observed, implying $S\sb0$ $\le$ 6 $\times$ 10$\sp{-5}$. The adsorption and desorption behavior of these molecules can be understood by regarding the Ge(100) dimer atoms as being linked by a strained double bond and examining analogous addition and elimination reactions of molecular germanium compounds.